If you encoded to Nero Digital before burning, you will get this happening when you attempt to burn to DVD - when it happened to me, I just stopped encoding to Nero Digital, and let the program transcode normally for your avi or whatever format files.
Result? Great DVD's
I don't know why Nero Digital does that, but it does. Maybe something to do with the difference between PAL and NTSC formats. Maybe another more knowlegable geek knows the answer!!